1. Field of the Invention
This invention relates to relief valve systems for sealed bearing roller cone rock bits.
More particularly, this invention relates to a vent cap pressure relief valve for sealed bearing roller cone rock bits, the valve relieving both low and high gas pressure differentials in stages during operation of the bit.
A rotary rock bit generally consists of a main bit body adapted to be connected to a rotary drill string. A conventional sealed bearing bit usually includes two or more legs integrally connected to form a bit body. Each leg includes a cutter cone rotatively mounted on a journal pin extending from the leg. Bearings are provided between the cutter and the journal pin to promote rotation of the cutter and means are provided on the outer surface of the cutter cone for cutting the formations in a borehole as the bit and cutter rotate.
In lubricated rock bits, a lubrication system is provided which includes an annular seal located at or near the back face of the cutter to prevent the lubricant from leaking from the bearing area to the exterior of the rock bit and to prevent drilling fluid and debris from entering the bearing area. The lubrication system further includes a reservoir filled with lubricant which is typically a high viscosity petroleum grease with passages provided to communicate the reservoir with the bearing space between the cutter cone and the journal pin. A compensator in the form of a resilient bootlike membrane is located within the reservoir with one side of the boot exposed to the lubricant and the other side of the boot exposed to the exterior of the rock bit. The compensator functions to equalize the pressure on the mud side of the seal with the pressure on the lubricant side under varying pressure conditions.
Sealed bearing bits, when operating at high RPM's (revolutions per minute) and at great depths sometimes overheat. The overheating may be attributed to a number of conditions. The bearings may be worn and overheat. Foreign detritus material may penetrate the bearing seals and cause the overheating problem or the rock bit may be abused by the drilling crews. In any event, when the bit overheats, the bearing grease stored in the rock bit reservoir may gasify. Gas under high pressure must have some way to escape the rock bit before damage to the roller cone seals occurs. Once the seals are destroyed, catastrophic failure of the entire rock bit follows shortly thereafter.
The vent cap relief valve of the instant invention provides a two-stage device to relieve gas pressure before the roller cone seals may be destroyed.
2. Description of the Prior Art
U.S. Pat. No. 4,161,223, assigned to the same assignee as the present invention, describes and teaches a single-stage means to relieve high gaseous pressure through a pressure relief valve.
The lubrication system incorporated within the sealed bearing rock bit includes a reservoir of lubricant, communicating via passageways within the bearing area as heretofore described. The reservoir further includes a rubber boot molded around a metal stiffener sleeve. A cover cap is attached to the rubber boot. The rubber boot is in the form of a resilient membrane and is exposed through the cover cap to the exterior of the rock bit and through the inner passageways to the interior of the lubrication bearing area formed between the roller cutter cone and a journal bearing extending from a leg of the rock bit. The vent and pressure relief system comprises an annular seat formed in the back wall surface of the rock bit. A valve face is formed on the rubber boot and is biased against the annular seat in the bit by means of a belleville spring acting on the cover cap. If any excessive pressure in this single-stage valve develops within the lubricant reservoir, the excess pressure is blown off through the single-stage valve seat. An additional feature includes a means in which any internal pressures within the bit may be manually vented through the valve without removing the cover cap by manually prying the cover cap off its seat.
A disadvantage in the foregoing patent is evident in that the valve is actuated only during relatively high gas pressures. Hence the seals protecting the bearing surfaces within the rock bit are subjected to excessive pressures just prior to actuation of the valve. The present invention obviates the excessive pressures subjecting the seals to stress by relieving lower pressures through a first stage valving action which does not lift the cover cap off the main valve seat but rather relieves a segment of the rubberlike seal around the periphery of the seal by allowing a portion of the seal to be depressed into a cavity or annular channel formed in the cover cap flange area above the main valve seat. Where gas pressures suddenly become very high, the seal will ultimately relieve in a second stage by lifting the entire cover cap against a belleville spring off its valve seat as taught by the prior art patent. The ability to relieve lower pressures through only a segment of the cover cap without actuating the main high pressure valve seat prevents any possibility of damage to any of the seals protecting the bearing surfaces interposed between the roller cones and the journals of the rock bit.